This invention relates generally to expansion valves used in refrigeration sytems and more particularly to an expansion valve which utilizes a stepper motor to control an adjustable orifice between the inlet and outlet ports of the valve.
The control of an expansion valve adjustable orifice by electronic rather than more direct thermostatic means is not in itself new and there are particular advantages in using a stepper motor to control the valve opening.
A stepper motor actuated valve provides discrete segments of valve stroke for each incremental angular movement (step) of the motor and has the advantages of being easily incorporated into a digital control system because the reliability and low hysterisis provide precise repeatable valve positions which can eliminate costly feed back loops. Such a valve is disclosed in U.S. Pat. No. 4,523,436 and utilizes a pair of telescopically related sleeves. The interior sleeve is stationary and includes an elongate sidewall orifice. The external sleeve is movable by a lead screw which is non-rotational and is driven in a linear direction by a stepper motor. In this telescopic expansion valve the lead screw does not rotate. However, there may be torsional frictional engagement between the lead screw motor when the valve bottoms out. Also, while such stepper motors are capable of precise control within the stroke limits, the requirement of a non-rotatable lead screw, which is subject to linear movement only, is a disadvantage and the provision of telescopic sleeves is an added expense.
It is considerably less expensive and more convenient to provide a stepper motor drive for a rotatable lead screw since the lead screw can be directly connected to the stepper motor. However, when a piston type valve is used to close a circular valve port or orifice, the use of a rotatable lead screw tends to result in undesirable torsional frictional engagement between the lead screw and valve parts at valve closure. Such engagement can result in binding which requires additional motor force to overcome in order to release the valve.
Another approach is to use a stepper motor having a pinion mounted to the rotor which is used to drive a rack type actuating member to operate the piston. Unfortunately, in addition to the expense and complexity, the precise control which is available with a stepper motor utilizing a lead screw is lost since the resolution is considerably less with a rack.
The present stepper motor actuated expansion valve overcomes the above and other problems in a manner not disclosed in the known prior art.